1. Field of the Invention
The following disclosure relates generally to a document lighting unit and an image scanning unit for use in an image processing apparatus.
2. Description of the Background Art
Increased development activities on a light emitting diode (hereinafter referred as an “LED”) have led to a development of an LED element having an increased light intensity.
A typical LED element has favorable properties such as longer lifetime, higher efficiency, and monochromatic light (i.e., emitting of light energy within a very narrow wavelength band), which can be applied in a variety of lighting applications. Applications using an LED element include a document lighting unit for use in an image scanning unit provided in an image forming apparatus such as a digital copier and image scanner.
FIG. 1 shows a schematic view of an image forming apparatus 1000 including an image scanning unit. As shown in FIG. 1, the image forming apparatus 1000 includes an image forming section 100 and an image scanning section 200.
The image forming section 100 includes an image carrying member 111 in drum-shape, a charge roller 112 for charging the image carrying member 111, a developing unit 113, a transfer roller 114, a cleaning unit 115, a fixing unit 116, a light scanning unit 117, a feed cassette 118, a pair of registration rollers 119, a feed roller 120, a transport route 121, an ejection roller 122, and an ejection tray 123.
The charge roller 112 can be a corona charger, for example.
The feed cassette 118 stores a transfer sheet “S” and is detachable from the image forming section 100.
The light scanning unit 117 scans a surface of the image carrying member 111 with a laser beam (LB) based on document information transmitted from the image scanning section 200 so that an electrostatic latent image can be written on the image carrying member 111.
The image forming apparatus 1000 conducts an image forming as now described.
The image carrying member 111 made of a photoconductive material rotates in a clockwise direction, with an uniform angular velocity. During such rotation, the charge roller 112 charges the surface of the image carrying member 111 uniformly. Then, the light scanning unit 117 scans the surface of the image carrying member 111 with the laser beam LB to write an electrostatic latent image on the image carrying member 111.
The electrostatic latent image on the image carrying member 111 is developed by the developing unit 113 as a toner image with toners.
The transfer sheet S stored in an upper most position of the feed cassette 118 is fed to the feed roller 120, and an edge of the transfer sheet S is sandwiched by the pair of registration rollers 119. The pair of registration rollers 19 feeds the transfer sheet S to a transfer position, defined by the image carrying member 111 and the transfer roller 114, to transfer the toner image to the transfer sheet S by synchronizing a sheet feed timing of the transfer sheet S to the transfer position. The transfer sheet S receives the toner image from the image carrying member 111 at the transfer position by an operation of the transfer roller 114. Then, the transfer sheet S is fed to the fixing unit 116 to fix the toner image on the transfer sheet S, and is then ejected to the ejection tray 123 via the transport route 121 and the ejection roller 122.
After transferring the toner image, the surface of the image carrying member 111 is cleaned by the cleaning unit 115 to remove deposits such as toners and paper powders.
As shown in FIG. 1, the image scanning section 200 includes a contact glass 201, a first moving unit 203 having a first mirror 203a, a second moving unit 204 having a second mirror 204a and a third mirror 204b, a focus lens 205, and a line sensor 206.
As shown in FIG. 1, the first moving unit 203, the second moving unit 204, the focus lens 205, and the line sensor 206 are provided under the contact glass 201, for example.
When a document 202 is placed on the contact glass 201, a lighting device (not shown) provided to the first moving unit 203 illuminates the document 202.
A reflection light from the document 202 reflects at the first mirror 203a in the first moving unit 203, and then reflects at the second mirror 204a and the third mirror 204b in the second moving unit 204, and then goes to the focus lens 205, and focuses on the line sensor 206. The line sensor 206 functions as a photoelectric converter and conducts image scanning in main scanning direction.
As shown in FIG. 1, when scanning a document, the first moving unit 203 moves in a direction shown by an arrow with a velocity of V, and the second moving unit 204 simultaneously moves in the same direction with a velocity of ½ V to scan the document, wherein the velocity of ½ V is one half speed of the velocity of V.
To illuminate a document, a document lighting unit of an image scanning unit generally has a width which is substantially the same as a document width. Therefore, when LED elements are used for a document lighting unit, a plurality of LED elements are arranged in an array manner.
Although the LED element has favorable properties as above-mentioned, a further improvement is required for light intensity of the LED element so that the LED element can efficiently supply light intensity for a lighting device of an image scanning unit.
Therefore, an image scanning unit employing LED elements is mainly marketed as a low-speed scanning unit and compactness-oriented scanning unit, for example, whereas an image scanning unit employing a cathodoluminescent lamp is mainly marketed as a high-speed scanning unit and large-scale scanning unit.
FIGS. 2A and 2B show a background configuration for illumination. Such a configuration includes LEDs 101, a contact glass 102, and a document-contact face 103.
In FIG. 2A, a dotted-curve line represents a light intensity distribution of each LED element 101 in the main scanning direction, and a solid-curve line represents a light intensity distribution which synthesizes the light intensity distribution of each LED element 101 in the main scanning direction. In FIG. 2B, a solid-curve line represents a light intensity distribution of LED element 101 in the sub-scanning direction.
To decrease the above-mentioned drawback (i.e., light intensity) of LED element 101, an LED array is placed in a position which is closer to the document-contact face 103 to increase light intensity on the document-contact face 103, for example. However, such positioning leads to an uneven light intensity distribution in the main scanning direction as shown in FIG. 2A because of an effect of a light intensity distribution curve of the each LED element 101.
On the other hand, if the LED array is placed in a position which is far from the document-contact face 103, a light intensity on the document-contact face 103 may decrease because of diffusion of lights coming from each LED element 101.
In another case, a number of LED elements 101 is increased to increase a number density of the LED elements 101 in the LED array having a width which is substantially the same as the document width. In this case, the LED array emits light in a similar manner of a cathodoluminescent lamp, which is a bar-shaped light source.
However, light intensity distribution in the main scanning direction becomes uneven when such a bar-shaped light source is used. Specifically, light intensity around the center portion of the bar-shaped light source becomes a maximum strength, and light intensity decreases from the center portion to the edge portion of the bar-shaped light source.
This is caused by a so-called shading effect. Specifically, the shading effect occurs because an effective number of LED elements 101 that contribute to the light intensity around the center portion of the document-contact face is larger than an effective number of LED elements 101 that contribute to the light intensity around the edge portion of the document-contact face 103. In other words, the center portion of the document-contact face 103 receives light from two directions in the main scanning direction (i.e., right and left direction with respect to the center portion), but the edge portion of the document-contact face 103 receives light from only one direction.
Furthermore, the focus lens 205 is provided in the image scanning section 200 as shown in FIG. 1, and thereby a drop of light intensity at a peripheral area on the line sensor 206 becomes more significant with an effect of the fourth-power-of-cosine law.
To cope with the above-mentioned drawbacks, a background art uses a lighting unit which can uniformly conduct an efficient illumination by providing a light-guiding member having a substantially plate-shape.
In this case, at least one diffraction grating is attached to a face of the light-guiding member to achieve a uniform distribution of the light intensity in the main scanning direction. However, such a configuration having a diffraction grating on the light-guiding member (i.e., a transparent member) leads to an increase of a manufacturing cost. Furthermore, such a configuration may not cope with a drop of light intensity at a peripheral area which is associated to a focus lens.
Another background art uses a shading correction plate to cope with a drop of light intensity at a peripheral area. However, such a configuration leads to an increase in the number of parts.
Still another background art uses a configuration that slants an LED array with respect to a to-be-scanned document to cope with a drop of light intensity at a peripheral area. However, such a configuration may lead to a larger mechanism, and may not increase the light intensity.
FIGS. 3A and 3B show a configuration for illumination using a bar-shaped light source in a background art. Such a configuration includes a bar-shaped light source 301 such as a cathodoluminescent lamp, mirror-face members 302 or 302a, a document-contact face 303, wherein the mirror-face members 302 or 302a are shaped in a semi-cylindrical shape having a concaved mirror portion, and the document-contact face 303 contacts with a document to be scanned. A curve line 304 shows a light intensity distribution in a sub-scanning direction on the document-contact face 303. As shown in FIGS. 3A and 3B, the mirror-face members 302 or 302a reflects light emitted from the bar-shaped light source 301. The background art uses the bar-shaped light source 301 and the mirror-face member 302 or 302a to achieve an adequate light intensity distribution by illuminating document-contact face 303 broadly or by a combination of a plurality of planes in the mirror-face member 302 or 302a. 
As shown in FIGS. 3A and 3B, the background art increases light intensity by reflecting lights emitted from the bar-shaped light source 301 at the mirror-face member 302 or 302a shaped in a semi-cylindrical shape and focusing such lights on the document-contact face 303.
The semi-cylindrical shape includes an opening portion and a cross-sectional shape expressed by a quadric curve such as a circle, ellipse, parabola, and hyperbola, and has a length which is substantially the same as a length of the bar-shaped light source 301 or a length of the document-contact face 303 in the main scanning direction.
FIG. 4 shows a schematic configuration of a light-receiving element included in an image processing apparatus such as a digital copier and image scanner. FIG. 4 shows a configuration of a focus lens 305 and a light-receiving element 306.
As shown in FIG. 4, in the image processing apparatus such as a digital copier and image scanner, a light reflected from the document is received by the light-receiving element 306 via the focus lens 305. The light-receiving element 306 includes a CCD (charge coupled device), for example, and has a width of from 0.05 to 0.1 mm, which is relatively smaller. In case of a 1:1 image focusing, only the above-mentioned small area having the above-mentioned width can be scanned on the document-contact face 303.
When the light from the light source is focused sharply, the focused light has a relatively smaller focused area. Under such a condition, if the lighting unit cannot maintain an adequate lighting position due to some factors such as deviation of a mirror angle from an adequate angle, the light intensity distribution curve line deviates from its adequate position. This may result in a significant change of the light intensity to be received by the light-receiving element 306, and consequently may affect an image to be produced.
Although FIG. 4 shows a case using a 1:1 image sensor, similar constrains can be observed for a reduction optical system. For example, if an image size is reducingly focused on the light-receiving element 306 to one tenth of an image size on the document-contact face 303, a width of image size on the document-contact face 303 is only about 1 mm, for example. Therefore, the above-described drawback on the light intensity deviation may be also observed.
FIGS. 5A, 5B, 5C, and 5D show relationships between a change of light intensity distribution curve line and a scanning area.
FIGS. 5A and 5B show cases in which a width of the light intensity distribution curve line is relatively small, and FIGS. 5C and 5D show cases in which a width of the light intensity distribution curve line is relatively large. The term “relatively” is used because the width of the light intensity distribution curve line is compared with the width of the scanning area.
FIGS. 5A and 5C show cases in which an illumination is conducted under a normal condition. In case of a digital copier, the light receiving element 306 has a smaller width such as 0.1 mm, for example. Therefore, as shown in FIG. 5B, if the center portion of the light intensity distribution curve line deviates from the scanning area, the light intensity at the scanning area may decrease significantly.
Accordingly, as for an image forming apparatus (e.g., a digital copier and image scanner), a document lighting unit, which has the light intensity distribution curve line having a larger span in the sub-scanning direction as shown in FIGS. 5C and 5D, is preferred to maintain the light intensity at the scanning area at a stable level even if the center portion of the scanning area deviates from the original position.
Specifically, a light intensity distribution curve line preferably has a flat portion around a peak of the light intensity distribution curve (i.e., portions having less unevenness of the light intensity).
In a background art, a first width required for scanning (e.g., approximately 1 mm) and a second width required for coping with a mechanical error (e.g., at least ±1 mm deviation is set for an effective scanning area) is combined and set as a minimum length to secure a flat portion around a peak of the light intensity distribution. The flat portion of the light intensity distribution is not strictly limited to a design value which can be defined by determining a mechanical configuration. The flat portion of the light intensity distribution can include some variation of the light intensity within a practicably permissible range that can be compensated with an electrical correction method, for example.
When the image forming apparatus is used for processing a monochrome image, for example, a variation of the light intensity of about 30% is permissible. When the image forming apparatus is used for processing a color image, the above-mentioned variation of the light intensity is permissible around 12% to maintain a balance of the three primary colors within a correctable range.
As above-mentioned, a background art uses a lighting unit which can uniformly conduct an efficient scanning by providing a light-guiding member having a substantially plate-shape. In this case, as above-mentioned, at least one diffraction grating is attached to reflecting faces to achieve a uniform distribution of the light intensity in the main scanning direction.
However, such a configuration having a diffraction grating on a transparent member leads to an increase of a manufacturing cost. Furthermore, such a configuration may not produce a flat portion in the light intensity distribution in the sub-scanning direction if an arrangement is not maintained appropriately.